After a massive success with the most recent Steam Game Festival back in June, it's going to return for another round later this year in October. This is where Steam users get to play through a ton of limited-time demos, which originally started back in December 2019 to go along with The Game Awards.
From a post on the Steamworks Development group on Steam, the date is confirmed to be October 7 - 13. Valve mentioned in the announcement that they will soon open up the developer opt-in for the event, giving developers another chance to get a demo out there and get more eyes on their game. Developers don't have long, as the opt-in date is only open from between August 19 - 26.
Like last time it seems they will also show livestreams, Q&A sessions and more. To be eligible, games need to have a launch date set between October 13 2020 and May 1 2021. In that same announcement, Valve confirmed more Steam Game Festivals for 2021 but didn't go into any more detail on that point but it's good to know it will be a regular thing.
I had a lot of fun going over the various Linux demos last time, you can see a little overview here if you missed it.
Speaking to Valve press, they confirmed to GOL that more details will be announced "soon".